This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
In Long Term Evolution (LTE) systems where Single Carrier Frequency Division Multiple Access (SC-FDMA) is used for uplink (i.e., from a User Equipment (UE) to an evolved NodeB (eNB)), the uplink reception performance may be seriously degraded by a frequency offset between a carrier frequency of a signal received at the eNB and a frequency of the eNB's local oscillator. Likewise, the downlink reception performance suffers from a frequency offset between a carrier frequency of a signal received at the UE and the eNB's local oscillator.
To guarantee an acceptable reception performance, it is thus necessary to estimate and compensate the frequency offset which mainly results from a Doppler frequency shift due to UE mobility.
In prior art (referring to US 2013/0070874 A1 for example), the estimation of the frequency offset is done by making use of the fact that the frequency offset causes a phase change between two OFDM reference symbols.
For illustration, the downlink subframe structure and the uplink subframe structure for LTE systems will be described with respect to FIGS. 1 and 2. As shown in the Figures, each subframe consists of a slot #0 and a slot #1, and each slot consists of seven OFDM symbols in case the subframe has a normal Cylic Prefix. Although not shown here, those skilled in the art will appreciate that each slot consists of six OFDM symbols in case the subframe has an extended CP.
More specifically, as illustrated in FIGS. 1 and 2 for the normal CP case, symbols #0, #4, #7 and #11 in a downlink subframe are used as reference symbols for channel estimation while the other symbols in the subframe are used as data symbols, and symbols #3 and #10 in an uplink subframe are used as reference symbols while the other symbols are used as data symbols. Likewise, for the extended CP case, both downlink and uplink subframes contain reference and data symbols.
According to the prior art frequency offset estimation approach US 2013/0070874 proposes, a phase change Ødiff ε[−π,π) between two reference symbols are measured at first. Then, a preliminary frequency offset fm_est is calculated as
            f      m_est        =                  ∅        diff                    2        ⁢        π        ⁢                                  ⁢                  t          d                      ,where td denotes a time distance between the two reference symbols. By way of example, for a downlink subframe as illustrated in FIG. 1, the two reference symbols may be selected as symbols #4 and #7. In this case, td is equal to 0.215 ms. For an uplink subframe as illustrated in FIG. 2, the two reference symbols may be selected as symbols #3 and #10. Accordingly, td is equal to 0.5 ms.
Based on the preliminary frequency offset fm_est, a plurality of frequency offset candidates can be determined as fn,offset=fm_est+n×fes, where nε{0, ±1, ±2 . . . } and fes denotes an observation frequency which may take a value of 1/td. Then, the received OFDM signal is decoded multiple times, with one of the plurality of frequency offset candidates applied to the received OFDM signal each time. In case the received OFDM signal is successfully decoded when a specific one of the plurality of frequency offset candidates is applied thereto, the frequency offset is determined as the specific frequency offset candidate.
One of the drawback of the prior art frequency offset estimation approach is that it cannot be applied to estimate the frequency offset for uplink reception in case frequency hopping is employed in the uplink. This is because, in the case of frequency hopping where different subcarriers are allocated to one UE in different slots, the phase change between the reference symbols #3 and #10 shown in FIG. 2 is no longer equal to 2πtdfm_est.
As another drawback of the prior art approach, the multiple attempts of decoding the received OFDM signal, to which the plurality of frequency offset candidates are respectively applied, consume large amounts of computation and power resources. The requirement for large amounts of computation resources may adversely incur high costs for the eNB and the UE. The consumption of large amounts of power resources may significantly reduce the UE's battery life.